1. Field of the Invention
The present invention relates to semiconductor integrated circuits and specifically to the integrated circuit devices used in switching power supplies with minimum numbers of pinouts.
2. Description of the Prior Art
The cost to manufacture a power supply is a principal concern in selecting between power supply types to be used in particular applications and the components selected to construct them. Switching power supplies have become cost competitive with much simpler linear power supplies because integrated circuit (IC) technology has advanced such that a majority of the complex switch mode circuits can be integrated on a single chip. As a rule of thumb, the fewer necessary pins on an IC package the lower will be the cost. A goal in switching power supply IC cost reduction has been the three-terminal package. One pin for power switching, one pin for ground and one pin for feedback. Conventional power supply ICs typically require as many as eight pins, thus limiting packaging options to the more expensive types.
FIG. 1 illustrates a prior art power supply 10 that includes a full-wave bridge rectifier 12, a filter capacitor 14, a transformer 16 with a primary winding 18 and a pair of secondary windings 20 and 22, a diode 24 and a capacitor 26 that provide DC output, a diode 28 and a capacitor 30 that provide a voltage Vbias, a compensated error amplifier 32, an opto-isolator 34, a four-pin switched mode power supply chip 36 and a Vbias smoothing capacitor 38 located near chip 36. Vbias provides power to operate chip 36. However, at power start-up, chip 36 will be without power because primary winding 18 will be open and no voltage will be induced into secondary winding 22 because chip 36 is not switching. To initiate such switching, a high voltage is tapped from the junction of a high voltage MOSFET 40 and the end of primary winding 18. This tap can supply enough current to start chip 36. A voltage regulator internal to chip 36 includes a high voltage preregulator transistor 42, a pass transistor 44 and an undervoltage comparator 46. During initial power-up when the voltage (Vs) across capacitor 38 is zero or near zero, a resistor 48 biases transistor 42 on and will pass current to charge up capacitor 38. When voltage Vs exceeds a predetermined threshold, comparator 46 will switch on transistor 44 and switch off transistor 42. Comparator 46 includes hysteresis, transistors 42 and 44 will not switch back unless the voltage Vs falls below a second threshold voltage that is lower than the first threshold voltage. With voltage Vs and transistor 44 on, power is supplied to a pulse width modulator (PWM) 50. This allows normal switching operation to begin and transistor 40 chops the current flowing through primary winding 18. Transformer 16 therefore begins operating and a voltage (V.sub.bias) develops from secondary winding 22, diode 28 and capacitor 30. With V.sub.bias being supplied across capacitor 38, comparator 46 operates to keep transistor 42 off and no further high voltage power is required. A feedback signal, proportional to the DC output, is returned by way of amplifier 32 and opto-isolator 34 to PWM 50. Chip 36 therefore requires a minimum of four pins, e.g., OUT, FB, VS and GND.
In the prior art, the need for a minimum of four terminals prevents the use of low cost three-terminal packages such as industry standards TO-22O, DPAK, TO-5, and equivalents. A three-terminal switched mode power supply chip is therefore needed by the industry.